The present invention relates to paper machines.
In particular, the present invention relates to the pickup and press section of a paper machine.
The present invention relates particularly to a press section wherein the paper web is conducted through successive press nips with so-called closed conduction according to which the web is continuously supported without any open draw, the web being supported at least through three successive press nips beyond the wire while engaging at all times either a felt or a roll, with dewatering of the paper web taking place as it travels through the pickup and press section.
As is well known, after a web undergoes an initial formation on the wire of a paper machine, this paper web has a considerable water content which may be approximately four times the fiber content of the web. However, at the stage when the web has travelled just beyond the couch roll and is about to be transferred from the wire by the pickup roll, the fiber structure of the web has not yet sufficiently solidified and the web does not yet have sufficient strength to resist strains to which the web will be subjected during subsequent operation thereon in the paper machine. The web is therefore conducted beyond the wire through the press section of the paper machine while passing through successive press nips defined between pairs of press rolls, and at each of these successive press nips the water content of the web is reduced while the strength thereof is increased. This type of mechanical drainage of water from the web influences in many ways both the operation of the paper machine and the properties of the finished paper. Those properties which are in particular influenced by the wet pressing are tensile strength, density, porosity, etc.
It is known that there is both a theoretical and a practical limit to the extent to which the web can be provided with a dry content as a result of the wet pressing. Beyond this limit dewatering must be carried out by evaporating procedures.
Of course, mechanical dewatering by pressing in the press section is, up to a certain point, considerably less expensive than evaporation of a corresponding amount of water in the drying section of the paper machine. It is highly desirable, therefore, to carry out as effective a dewatering as possible in the press section, so that as a result the paper web is subjected to a relatively intense pressing at the press nips in the press section.
However, because of the fact that the wet web detached from the wire is relatively weak, there are certain limits to the pressure which can be exerted on the web to extract water therefrom. If this pressure is too great the network of fibers which form the paper web will be deleteriously influenced. Furthermore, in order to prevent any detrimental effects which the mechanical pressing may have on the quality of the paper, a considerable amount of care must be utilized in selecting the combinations of rolls which are utilized in the press section of the paper machine in order to secure as effective a water removal as possible without danger of an undesirable decrease in the quality or strength of the finished paper.
It is conventional to utilize in the press section of a paper machine a press nip defined between a pair of press rolls one of which usually has a smooth surface while the other is a vented nip type of roll having a recessed surface, and a felt also travels through this type of nip. The paper web is conducted into a nip of this latter type in such a way that it travels between the felt and the surface of the plain roll. The felt functions to contribute to draining off water which is pressed out of the paper web as well as to reduce any tendency to form on the paper web a pattern or marking which will correspond to the recessed surface of the vented nip roll.
Moreover, it is conventional to provide only one felt at each press nip. The result of this latter type of arrangement is that dewatering from the web takes place in one direction only, namely toward the felt. Such dewatering in one direction only has the drawback of contributing toward an asymmetrical web structure because the fines and fillers in the paper web easily travel together with the draining water toward the felt and thus will accumulate at one surface only of the paper web. In order to avoid this latter undesirable lack of symmetry in the structure of the paper, at a press nip where a considerable amount of water is to be extracted from the web and where there is a particular risk that the paper web structure will be undesirably skewed or in other words will become undesirably asymmetric, it is possible to provide a pair of felts so that the paper web passes through the nip while engaging a pair of felts at the opposite surfaces of the paper web. With such a press nip it is advantageous that both of the press rolls be vented nip types of rolls having recessed surfaces, which of course means that such rolls may take the form of suction press rolls having perforated shells or they may be formed with bores having closed inner ends and extending inwardly from the exterior surface of the roll, or the rolls may be grooved, as is known in connection with vented nip rolls.
This type of pressing which takes place between two felts, in other words double-felted nip type of pressing, is of particular advantage with respect to the dewatering action. In some cases the extent of dewatering is facilitated in this way to such a great extent that the desired dry-matter content of the web can be achieved while utilizing smaller pressing forces than in a nip where only a single felt is provided. The result is that the web structure is strained to a lesser extent. On the other hand, in a double-felted nip, as contrasted with a single-felt nip, it is possible to increase the pressure between the rolls without running the risk of damaging the web. Double-felted press nips have already been proposed. In this connection reference may be made to the copending U.S. patent application Ser. No. 310,805, now abandoned. As is disclosed in the latter application a single double-felted press nip is provided while the subsequent nips are only single-felt types of nips. In the case where only the first of the series of press nips of the press section is double-felted, while the several subsequent nips are only single-felted, there is a possibility that the uniform, symmetrical distribution of the material in the web as achieved at the first double-felted nip will be disturbed and spoiled as a result of the bias which occurs at the following single-felt nips where dewatering takes place in one direction only. However, such subsequent single-felt nips as well as additional nips are required in order to be able to achieve a sufficient dryness of the web which has travelled through the press section.
In addition to the above problems in connection with achieving in the press section both a desirable degree of dryness in the web and a uniform structure throughout the thickness of the web, particular problems are encountered with certain types of paper particularly paper which is to have a highly porous structure, as well as paper and paper board of relatively thick grades, and also in connection with creped paper.